Recently, with the spread use of color printers, color scanners, and the like, many colored documents are generated. This increases the opportunity to capture these documents by scanning and save them as electronic files or transmit them to the third parties through the Internet or the like. Handling these data as full-color data, however, imposes a heavy load on a storage unit or line. For this reason, the amount of data to be handled must be reduced by a method like a compression processing method.
Conventional methods of compressing color images include a method of compressing an image into a pseudo-grayscale binary image by error diffusion or the like, a method of compressing an image in the JPEG form, a method of converting an image into 8-bit palette color image and ZIP- or LZW-compressing the image (e.g., Japanese Patent Laid-Open No. 2002-077631).
Although conversion to a pseudo-grayscale binary image reduces the size (data amount), color information is lost. Compression in the JPEG form, however, causes a tradeoff between compression size and character quality.
According to the method of converting an image into a palette color image and ZIP- or LZW-compressing it, since most color distributions of a color document image are not discrete but are local, holding the image in a multi-bit form results in poor efficiency. As a consequence, this image is compressed with poor efficiency.
According to the method disclosed in Japanese Patent Laid-Open No. 2002-077631, a combination of area determination, binary compression by MMR, lossless compression by ZIP, and lossy compression by JPEG makes it possible to obtain high quality with respect to a general character area. However, area determination is difficult to performed with respect to a portion of a document to which correction is made by handwriting. Such an area is subjected to JPEG compression to result in mosquito noise.